1. Field of the Invention
This invention relates to an apparatus attaching a unit. More specifically, this invention relates to an apparatus equipped with a detachable unit.
2. Description of the Related Art
An image forming apparatus, for example a MFP (Multifunction Peripheral), is equipped with a detachable unit for consumables (a consumable unit), for example a toner bottle, an imaging unit, a photo conductor, a developing device or the like. When the consumable unit reaches the end of life which is predetermined (for example, a life time, and a durability criterion of the number of prints), the consumable unit should be replaced with a same type new consumable unit. To determine whether the consumable unit reaches the end of life or not, a conventional image forming apparatus has a function for detecting the amount of the consumable unit use, a function for detecting an attachment (equipment) of a new consumable unit to the image forming apparatus, and a function for determining whether the consumable unit is appropriately installed on the image forming apparatus or not.
When the conventional image forming apparatus determined that the consumable unit is appropriately loaded on the image forming apparatus, the image forming apparatus determines whether the loaded consumable unit is new or old (used one) by detecting electrically the melting and blowing state of a fuse installed on the circuit board in the consumable unit. More specifically, when the fuse melted and blew, the conventional image forming apparatus determines that the consumable unit is old one. On the contrary, when the fuse does not melt, the conventional image forming apparatus determines that the consumable unit is new one and makes the fuse melt and blow.
For example, documents 1 and 2 below disclose a technique distinguishing between an old consumable unit and a new consumable unit installed, by detecting electrically the melting and blowing state of the fuse installed on the circuit board in the consumable unit.
Document 1 below discloses an image forming apparatus having a measuring means for measuring the operating time of an exchangeable component which has an electrical current blocking means. The image forming apparatus comprises a conduction state detecting means, a reserve means, a comparing means, a new one determining means, and a measuring control means. The conduction state detecting means detects the conduction state in the electrical current blocking means at a prescribed timing. The reserve means reserves the detected electrical voltage levels. The comparing means compares the electrical voltage level detected by the conduction state detecting means at the next timing and the electrical voltage level reserved. The new one determining means determines that a new replaceable component is loaded, in case that the electrical voltage level detected is the same as that of a new component, on the result of the comparing. The new one determining means determines that the same replaceable component is loaded, in case that the electrical voltage level detected is not the same as that of a new component and is the same as that of the electrical voltage level reserved, on the result of the comparing. The measuring control means resets the measured value of the measuring means and blocks the electrical current in the electrical current blocking means, in case that the apparatus determines that the new replaceable component is loaded, based on the result of the new one determination.
Document 2 below discloses a unit detection apparatus having a unit which includes a unit side circuit, and a main body of an image forming apparatus which includes a main body side circuit. The unit side circuit has a fuse and a first resistor which is parallely connected to the fuse. The main body side circuit includes a power supply, a first circuit, a second circuit, a detector, and a melt and blow circuit. The first circuit has a second resistor being connected to the power supply via the unit side circuit, when the unit is loaded on the main body of the image forming apparatus. The second circuit has a third resistor and a switching device being series-connected to the third resistor. The second circuit is parallely connected to the second resistor. The detector detects the electrical voltage or electrical current at the prescribed location in the first circuit. The melt and blow circuit turns the switching device off during the predetermined period and melts and blows the fuse, in case that the circuit determines that the fuse in the unit side circuit does not melt and not blew, based on the detecting result of the detector.    Document 1: Japan Patent Publication No. 2010-217540    Document 2: Japan Patent Publication No. (Hei) 11-143304
However, the conventional image forming apparatus has a problem that the new or old determination result of a consumable unit is less reliable. More specifically, in case that an abnormal circumstance (a short circuit) occurs at a connection part between the circuit board of the image forming apparatus side and the circuit board in the consumable unit, the fuse may mistakenly melt and blow by electrical voltage applied for detecting the melting state of the fuse. In particular, a sheet metal plate may be installed around a contact point of the circuit board of the image forming apparatus, to suppress static electric noise. In this case, when the contact point deformed etc., a short circuit is likely to happen at the connection part between the circuit board of the image forming apparatus side and the circuit board in the consumable unit. In consequence, the image forming apparatus may mistakenly determine that the new consumable unit is old, after recovering from the abnormal circumstances.